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Are sugars and starches dangerous for rabbits?

In the autumn, many apples can fall from trees as windfalls. These are very palatable for rabbits. They will feat on them and eat the whole apple, including the pips, without ill effects. There are no confirmed reports of  illness in rabbits caused by apple pips.

A current trend in the rabbit world is to believe that sugars and starches are dangerous for rabbits because they can cause ‘caecal dysbiosis’ and many owners avoid feeding fruit or root vegetables because of this. These foods are not dangerous and rabbits enjoy eating apples and carrots. They do not need to be excluded from the diet but should be fed in moderation because roots vegetables and fruit are fattening and calcium deficient.

What is caecal dysbiosis?

'Caecal dysbiosis’ means a disruption in the balance of micro-organisms in the caecum, which is a contributory factor to enterotoxaemia. Enterotoxaemia is the reason why some antibiotics are dangerous for rabbits if they are given orally. They can selectively kill some bacteria and allow others (such as Clostridial species) to proliferate.

Carbohydrate overload theory

The relationship between fibre, starch and enteris disease is important for commercial rabbits because looses due to enteritis anr high. It has been the subject of many studies. A review article can be found here.

The idea that sugars and starches are dangerous for rabbits probably originates from the ‘carbohydrate overload’ theory that originates from outbreaks of enteric disease in rabbits bred for intensive meat product. It doesn't apply to pet rabbits. 
The carbohydrate overload theory is based a type of bacteria,  Clostridia,  that is often  found in outbreaks of enteritis in rabbit colonies. This bacterium causes a serious, often fatal, diarrhoea (enterotoxaemia). The subspecies that is often found in rabbits, Clistrium spiriforme,  requires glucose for toxin production. The  idea of the carbohydrate overload theory is that a diet that is high in starch provides glucose to the caecal microflora for toxin production and this results in enterotoxaemia. Even in the scientific community of commercial rabbit nutrition, the theory is not universally accepted and even if it were true, it would only apply to young rabbits without an unbalanced caecal microflora. In adult rabbits, sugars and starches are absorbed in the small intestine. Also, a high carbohydrate diet tends to be low in fibre and this is more likely to be the reason that commercial rabbits are so susceptible to enteric disease. They also live in crowded conditions where they are stressed and exposed to high envoronmental loads of bacteria and other microorganisms, such as coccidia that can also cause enteritis. 

Carbohydrates and pet rabbits

In contrast to commercial rabbits, pet rabbits are usually adults that live on their own, in pairs or in small groups. Crowded, insanitary conditions are unusual  Unlike commercial rabbits, they are not exposed to enteric infections from other rabbits or from a contaminated environment.  Enterotoxaemia is rare despite the fact that most pet rabbits are fed on a diet that contains starches. A look at the ingredients of muesli mixes, nuggets or pellets will show that they contain oats or wheat i.e carbohydrates. Some contain sugary molasses.  Common sense would say that  sugary, starchy foods are highly unlikely to kill an adult pet rabbit by upsetting its caecal microflora otherwise there would be a lot of dead pet rabbits about. The rabbits that live on muesli mix and honey flavoured treats from the pet shop do not die from enterotoxaemia. They suffer from obesity instead. High calorie treats and muesli mix are never a good idea for rabbits.  Unfortunately, some rabbit food manufacturers still produce and market them

Caecl dysbiosis and soft caecotrophs

The term 'caecal dysbiosis'  is often used in relation to an idea that high dietary starch causes soft caecotrophs by altering the balance of micro-organisms within the caecum so the caecotrophs are soft and sticky. This is unproven and unlikely as it does not fit with digestive physiology. In rabbits, simple monosaccharide sugars, such as glucose, fructose and galactose, are absorbed from the small intestine in a similar manner to other species. Starches in seeds, fruits, tubers and roots and are broken down to simple sugars during digestion and, as in other species, this reaction is catalysed by amylase secreted by the salivary glands and pancreas. In rabbits, the pancreas is not fully developed until about 6 weeks,  so in young rabbits. some dietary starch may reach the caecum. In adult rabbits, nearly all the starch is broken down and absorbed as sugars in the small intestine, so very little reaches the caecum.  Studies have shown that dietary starch has been shown to have no influence on the chemical composition of caecal contents or on the production or composition of soft and hard faeces. Any link between high carbohydrate diets and soft caecotrophs is probably due to the indirect relationship of carbohydrate with fibre. High carbohydrate foods (such as fruit and cereals) tend to be low in indigestible fibre. The fibre content of caecotrophs affects their consistency, more fibre makes making them firmer. This is why a high fibre diet is a good remedy for soft caecotrophs. A high fibre diet is less fattening, so the rabbit is thinner and more able to reach round to consume the caecotrophs.  The rabbit will be hungrier and keener to eat caecotrophs. If the caecotrophs are ingested, their consistency is unknown.